Polarizer, projection lens system, exposure apparatus and exposing method

ABSTRACT

The directions of amplitude of polarized light passing through a polarizer are concentric around a position. The polarizer is disposed on the surface of a pupil such that the position lies exactly on the center of the surface of the pupil. Rays of luminous flux of illumination light converted into polarized light by the polarizer are converged onto a wafer with concentric planes of polarization with respect to an optical axis. The illumination light is therefore incident on a photoresist as s-polarized light. Thus, the amount of light entering the photoresist is less likely to depend upon the angle of incidence. Consequently, the contrast of an optical image formed in the photoresist is improved, and hence, resolution characteristics are improved.

RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.10/883,007, filed Jul. 2, 2004, claiming priority of JapaneseApplication No. 2003-190205, filed Jul. 2, 2003, the entire contents ofeach of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for manufacturing asemiconductor device, and more particularly to a technique for exposure.

2. Description of the Background Art

Semiconductor devices are continuously becoming miniaturized, and thenumerical aperture (NA) of exposure apparatuses is being increased forfurther miniaturization.

However, as NA is getting higher, the angle of incidence on a target ofexposure, e.g., a photoresist increases, which causes the effects ofpolarization which have been neglected to become evident. This causesdegradation in imaging characteristics in the photoresist. Suchcircumstances are introduced in “Challenges in high NA, polarization,and photoresists” by Bruce W. Smith, et al., SPIE 2002, 4691-2, pp.11-24. The following documents are related to the present invention:“Polarizing & Retardation Films” retrieved on May 15, 2003 from theNitto Denko homepage<URL:http://www.nitto.co.jp/product/industry/electronics/output/lcds/polar/index.html>;Japanese Patent Application Laid-Open No. 5-226225 (1993); and JapanesePatent Application Laid-Open No. 2001-185476.

Optical systems of usual exposure apparatuses have not been able tocontrol polarization of diffracted light generated through a maskpattern, so that polarized light incident on a photoresist includesp-polarized light and s-polarized light in an even ratio. With anincrease in NA of exposure apparatuses, the angle of incidence on atarget of exposure, i.e., a photoresist increases as described above,causing the ratio of p-polarized light and s-polarized light incident onthe photoresist to vary. Further, since the contrast of an optical imagediffers between p-polarized light and s-polarized light, the contrast ofan optical image generated by composition of p-polarized light ands-polarized light is degraded. Then, improvement in the degree ofresolution cannot be expected even where NA is increased.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the contrast of anoptical image generated in a photoresist, and therefore to improveresolution characteristics.

According to the present invention, a polarizer includes an even numberof regions arranged adjacently to one another around a center.Directions of amplitude of light passing through the even number ofregions distribute in the form of a regular polygon with an even numberof members. Alternatively, the polarizer includes a plurality oftransmission regions having a common center. Directions of amplitude oflight passing through the plurality of regions distributeconcentrically.

The polarizer is disposed with its center lying on the center of thesurface of the pupil of a projection lens system in an exposureapparatus, thereby transmitting s-polarized light of illumination lightin the exposure apparatus while cutting off p-polarized light. Thus,exposure using light which has passed through the polarizer can improvethe contrast of an optical image, and hence, resolution characteristicscan be improved.

According to the present invention, a projection lens system employs thepolarizer as a pupil filter.

Therefore, variations in exposure characteristics of an exposureapparatus can be reduced, even when an increase in the numericalaperture of the projection lens system widens the range of distributionsof the angle of incidence.

According to the present invention, an exposure apparatus employs theprojection lens system.

Therefore, exposure using s-polarized light of illumination light canimprove the contrast of an optical image, and hence, resolutioncharacteristics can be improved.

According to the present invention, an exposure method uses a mask witha light shielding portion and a pupil filter. A direction in which thelight shielding portion extends is in parallel to a direction ofamplitude of polarized light transmitted through the pupil filter.

S-polarized light of illumination light is transmitted at exposure,while p-polarized light is cut off. Therefore, exposure using lightwhich has passed through the polarizer can improve the contrast of anoptical image, and hence, resolution characteristics can be improved.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical system of a projection exposure apparatus towhich the present invention is applicable;

FIGS. 2 and 3 show the direction of amplitude of incident light;

FIG. 4 is a graph showing the reflectance of incident light;

FIG. 5 is a graph showing the light intensity in a photoresist;

FIG. 6 conceptually shows the behavior of light transmitted through amask;

FIG. 7 conceptually shows positions where light passes through thesurface of pupil;

FIG. 8 shows the characteristics of a polarizer according to a firstpreferred embodiment of the present invention;

FIGS. 9 to 11 show the characteristics of a polarizer according to asecond preferred embodiment of the invention;

FIGS. 12 and 13 show the characteristics of a polarizer according to athird preferred embodiment of the invention; and

FIGS. 14 to 16 show the characteristics of a polarizer according to afourth preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Preferred Embodiment

FIG. 1 shows an optical system of a projection exposure apparatus towhich the present invention is applicable. Illumination light emanatedfrom a lamp house 11 is reflected off a mirror 12, and passes through afly eye lens 13, an aperture member 14, a relay lens 15A and a blind 16in this order, and is further reflected off a mirror 17.

The fly eye lens 13 is divided into a plurality of lens regions 13 a,and light emitted from the respective lens regions 13 a pass through anopening of the aperture member 14. On the surface of a photomask 18, thelight rays from the individual lens regions 13 a are laid on top of oneanother, which means the lens regions 13 a contribute to uniformillumination.

The illumination light reflected off the mirror 17 reaches the photomask18 with a circuit pattern formed thereon, through a condenser lens 15B.The light which has passed through the photomask 18 (includingdiffracted light) passes through a projection lens system 19 to reach awafer 20. A photoresist not shown is provided on a surface of the wafer20 that faces the projection lens system 19 and is exposed to theillumination light which has passed through the projection lens system19.

The projection lens system 19 includes an aperture stop 22, and apolarizer 21 for transmitting s-polarized light and cutting offp-polarized light is provided as a pupil filter on the surface of thepupil of the projection lens system 19. Advantages of transmittings-polarized light in the present invention will be discussed now.

FIGS. 2 and 3 schematically show the directions of amplitude J of lightin the case where incident light Q incident on a target is p-polarizedlight and s-polarized light, respectively. When the incident light Q isp-polarized light, the directions of amplitude J of light are inparallel to the plane decided by the direction of the normal R to thetarget and the direction of the incident light Q, as shown in FIG. 2.When the incident light Q is s-polarized light, the directions ofamplitude J of light are perpendicular to that plane, as shown in FIG.3.

FIG. 4 is a graph showing the reflectance of incident light Q where atarget is a photoresist. The angle of incidence θ made by the directionof the normal R and the direction of the incident light Q is plotted onthe horizontal axis, and the reflectance is plotted on the verticalaxis. Dependence of the reflectance on the angle of incidence θ differsbetween s-polarized light and p-polarized light. Particularly, thereflectance of p-polarized light greatly depends on the angle ofincidence θ. As NA increases, the angle of incidence θ has a widerdistribution. From this point of view, it is preferable to adopts-polarized light for exposure.

FIG. 5 is a graph showing the light intensity of an optical image in aphotoresist. Here, NA shall be 0.707, and the photoresist shall have anindex of refraction of 1.5. Such intensity distributions result frominterference of light that enters the photoresist. S-polarized light hasa steeper distribution than p-polarized light, and also from this pointof view, it is preferable to adopt s-polarized light for exposure.

Next, a desirable structure for the polarizer 21 to transmit s-polarizedlight while cutting off p-polarized light will be discussed.

FIG. 6 conceptually shows the behavior of light transmitted through thephotomask 18. At the time when incident light Li incident on thephotomask 18 passes through the edge of the pattern formed on thephotomask 18, diffracted light is generated in addition to zero-orderdiffracted light L0 that has not been diffracted. FIG. 2 only showsfirst-order diffracted light L1 primarily diffracted. The zero-orderdiffracted light L0 and first-order diffracted light L1 pass through thesurface of the pupil 22 a to reach the photoresist on the wafer 20.

FIG. 7 conceptually shows positions where light passes through thesurface of the pupil 22 a. FIG. 7 only shows diffracted light of onlyzero and first orders. The center 22 b of the surface of the pupil 22 a,a position P0 through which zero-order diffracted light passes and aposition P1 through which first-order diffracted light passes lie on astraight line M. Diffracted light of subsequent orders also lies on thesame straight line M.

Therefore, in order to make light transmitted through the polarizer 21s-polarized light, only polarized light that is perpendicular to linespassing through the center 22 b may be passed. For instance, thepolarizer 21 may only polarize light such that the directions ofpolarization are distributed concentrically around the center 22 b.

FIG. 8 schematically shows the directions of amplitude J of polarizedlight passing through the polarizer 21. The directions of amplitude Jare concentric with respect to a position D. This can be said that aplurality of transmission regions commonly centering at the position Dare provided, and the directions of amplitude of light passing throughthese regions distribute concentrically. For instance, a polyvinylalcohol film is dyed with and adsorbs iodine or organic dye and the filmis drawn, so that a filter for transmitting polarized light traveling ina predetermined direction is generated (see e.g., aforementioned“Polarizing & Retardation Films”), and the filter thus generated isdrawn isotropically with respect to its center. The polarizer 21 isthereby manufactured.

The polarizer 21 is disposed on the surface of the pupil 22 a such thatthe position D lies exactly on the center 22 b. The rays of luminousflux of illumination light which have passed through the polarizer 21are converged onto the wafer 20 with concentric planes of polarizationwith respect to the optical axis. That is, the illumination light isincident on the photoresist in the form of s-polarized light.

Through the use of such polarizer 21, the illumination light is incidenton the photoresist on the wafer 20 in the form of s-polarized light inthe projection lens system 19, so that the amount of light entering thephotoresist is less likely to depend upon the angle of incidence θ. Thiscan reduce variations in exposure characteristics of an exposureapparatus, even when an increase in NA of the projection lens system 19widens the range of distribution of the angle of incidence θ. Further,an optical image in the photoresist becomes steeper to improve itscontrast, which therefore improves resolution characteristics.

Second Preferred Embodiment

In the present embodiment, another mode of the polarizer 21 will bedescribed, by way of example. FIG. 9 illustrates a pattern on thephotomask 18. In the pattern, a plurality of light shielding portions 18a extending in one direction are arranged at intervals in parallel toone another. Light incident on the photomask 18 passes through theintervals, while the light shielding portions 18 a shield the incidentlight.

FIG. 10 schematically shows the behavior of diffracted light generatedby the photomask 18, seen in a direction parallel to that in which thelight shielding portions 18 a extend.

Incident light Li, after passing through the intervals between the lightshielding portions 18 a, is split into zero-order diffracted light L0and first-order diffracted light L1 (or higher-order diffracted light).Since the light shielding portions 18 a extend in one direction, thediffracted light has the direction of polarization having a componentparallel to that of the extending direction of the light shieldingportions 18 a except at the ends of the light shielding portions 18 a inits extending direction.

Therefore, as shown in FIG. 9, in exposure using the light shieldingportions 18 a extending in one direction, causing the polarizer 21 totransmit only polarized light traveling in the extending direction ofthe light shielding portions 18 a allows s-polarized light to betransmitted. FIG. 11 schematically shows the directions of amplitude Jof polarized light passing through the polarizer 21. The directions ofamplitude J of polarized light are aligned in one direction.

Disposing the polarizer 21 on the surface of the pupil 22 a (FIG. 6)such that the directions of amplitude of polarized light passing throughthe polarizer 21 are in parallel to the extending direction of the lightshielding portions 18 a can achieve the same effects as in the firstpreferred embodiment as to exposure using the light shielding portions18 a.

The effects of the present embodiment become more apparent as the lightshielding portions 18 a extending in one direction occupy a larger areaon the photomask 18. However, in the case where the pattern on thephotomask 18 extends in various directions, it is preferable to adoptthe polarizer 21 that makes the directions of amplitude of light passingtherethrough concentric, such as that shown in FIG. 8, by way ofexample, in the first preferred embodiment.

Third Preferred Embodiment

FIG. 12 schematically shows the directions of amplitude J1 and J2 oflight passing through the polarizer 21 according to the presentembodiment. The polarizer 21 is equally divided in to four regions S1 toS4 centering at a position D. The direction of amplitude J1 of polarizedlight passing through the regions S1 and S3 facing each other and thedirection of amplitude J2 of polarized light passing through the regionsS2 and S4 facing each other are perpendicular to each other.

Therefore, where there exist two pairs of light shielding portions inthe pattern on the photomask 18, each pair extending in one direction,and the extending directions of the two pairs cross each other at 90(=360/4) degrees, the same effects as in the second preferred embodimentcan be achieved. When disposing the polarizer 21 on the surface of thepupil 22 a (FIG. 6), it is preferable that the directions of amplitudeJ1 and J2 and the extending directions of the two pairs of lightshielding portions match, respectively.

FIG. 13 schematically shows the directions of amplitude J1 to J4 ofpolarized light passing through the polarizer 21 according to thepresent embodiment. The polarizer 21 is equally divided into eightregions S11 to S18 around the position D. The direction of amplitude J11of polarized light passing through the regions S11 and S15 facing eachother crosses the direction of amplitude J12 of polarized light passingthrough the regions S12 and S16 facing each other at 45 degrees. Thedirection of amplitude J13 of polarized light passing through theregions S13 and S17 facing each other crosses the direction of amplitudeJ12 at 45 degrees. The direction of amplitude J14 of polarized lightpassing through the regions S14 and S18 facing each other crosses thedirection of amplitude J13 at 45 degrees. The directions of amplitudeJ11 and J14 cross each other at 45 degrees.

Therefore, where there exist four pairs of light shielding portions inthe pattern on the photomask 18, each pair extending in one direction,and the extending directions of the four pairs cross one another at 45(=360/8) degrees, the same effects as in the second preferred embodimentcan be achieved. When disposing the polarizer 21 on the surface of thepupil 22 a (FIG. 6), it is preferable that the directions of amplitudeJ11 to J14 and the extending directions of the four pairs of lightshielding portions match, respectively.

Further considered is a general case in which the number of directionsof amplitude of polarized light passing through the polarizer 21 is Nand a minimum positive angle made by the respective directions ofamplitude is (360/2N) degrees. In this case, the polarizer 21 is equallydivided into 2N regions, and polarized light passing through regionsfacing each other has the same direction of amplitude. That is,distributions of the directions of polarized light passing through thepolarizer 21 form a regular polygon with 2N members.

The use of such polarizer 21 in the case where there exist N pairs oflight shielding portions in the pattern on the photomask 18, each pairextending in one direction, and the extending directions of therespective pairs cross each other at a positive minimum angle of(360/2N) degrees can achieve the same effects as in the second preferredembodiment.

Fourth Preferred Embodiment

FIG. 14 schematically shows the directions of amplitude J of polarizedlight passing through the polarizer 21 according to the presentembodiment. The polarizer 21 has a ring-shaped region S0 centering atthe position D. The region S0 is interposed between two circlesconcentric with respect to the position D. Selection of polarized lightis not conducted in the range surrounded by the region S0, but incidentlight is transmitted.

The directions of amplitude J of polarized light are concentric withrespect to the position D, similarly to the directions shown in FIG. 8.The polarizer 21 is disposed on the surface of the pupil 22 a such thatthe position D lies exactly on the center 22 b. The polarizer 21 mayconsist of the region S0 alone.

As shown in FIG. 4, where the angle of incidence θ is about 40 degreesor smaller, a difference in reflectivity between s-polarized light andp-polarized light increases with an increase in the angle of incidenceθ. Thus, p-polarized light resulting from incident light having a greatangle of incidence may be limitedly cut off. The region S0 shown in FIG.14 is a peripheral portion with respect to the center D, where the angleof incidence is great. Therefore, the use of the polarizer 21 accordingto the present embodiment can improve the contrast of illumination lightmade incident at such an angle that resolution characteristicssignificantly degrade, similarly to the first preferred embodiment.

FIGS. 15 and 16 each schematically show the directions of amplitude ofpolarized light passing through another type of polarizer 21 accordingto the present embodiment. FIG. 15 shows the structure in which theregions S1 to S4 of the polarizer 21 shown in FIG. 12 are transformedinto regions S21 to S24 limited in a range at a certain distance fromthe center D. FIG. 16 shows the structure in which the regions S11 toS18 are transformed into regions S31 to S38 limited in a range at acertain distance from the center D.

Even where the directions of amplitude of transmitted light are limitedin a ring-shaped region as in the present embodiment, the contrast ofillumination light made incident at such an angle that resolutioncharacteristics significantly degrade can be improved, similarly to thethird preferred embodiment.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1-17. (canceled)
 18. An exposure method using an optical systememploying a polarizer, wherein said polarizer include an even number ofregions arranged adjacently to one another around a center and isprovided on the surface of a pupil of said optical system, anddirections of amplitude of light passing through said even number ofregions distribute in the form of a regular polygon with an even numberof members.
 19. The exposure method according to claim 18, wherein saideven number of regions are arranged annularly.
 20. The exposure methodaccording to claim 18, wherein a light passing through said polarizer isincident on a photoresist over a wafer and expose said photoresist, andcontent of s-polarized light in said light incident on said photoresistis larger than content of p-polarized light in said light.